DAG: Fix expansion of DYNAMIC_STACKALLOC for StackGrowsUp targets

[arsenm]

Summary

Tests will be included with AMDGPU patch to start using this
expansion.

[arsenm]

Actually on second look, I can't use this implementation directly which I should have noticed before (and I'm also not sure how my test passes with this bug). This fix would still be needed for StackGrowsUp targets, but AMDGPU also needs to scale the increment amount by the wavesize which isn't done here

[rampitec]

LGTM

[arsenm]

dda82986f97747350dce4e8ebd65c27a64a37c9d

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

void SelectionDAGLegalize::ExpandDYNAMIC_STACKALLOC(SDNode* Node,
   // Chain the dynamic stack allocation so that it doesn't modify the stack
   // pointer when other instructions are using the stack.
   Chain = DAG.getCALLSEQ_START(Chain, 0, 0, dl);
 
   SDValue Size  = Tmp2.getOperand(1);
   SDValue SP = DAG.getCopyFromReg(Chain, dl, SPReg, VT);
   Chain = SP.getValue(1);
   unsigned Align = cast<ConstantSDNode>(Tmp3)->getZExtValue();
-  unsigned StackAlign =
-      DAG.getSubtarget().getFrameLowering()->getStackAlignment();
-  Tmp1 = DAG.getNode(ISD::SUB, dl, VT, SP, Size);       // Value
+  const TargetFrameLowering *TFL = DAG.getSubtarget().getFrameLowering();
+  unsigned Opc =
+    TFL->getStackGrowthDirection() == TargetFrameLowering::StackGrowsUp ?
+    ISD::ADD : ISD::SUB;
+
+  unsigned StackAlign = TFL->getStackAlignment();
+  Tmp1 = DAG.getNode(Opc, dl, VT, SP, Size);       // Value
   if (Align > StackAlign)
     Tmp1 = DAG.getNode(ISD::AND, dl, VT, Tmp1,
                        DAG.getConstant(-(uint64_t)Align, dl, VT));
   Chain = DAG.getCopyToReg(Chain, dl, SPReg, Tmp1);     // Output chain
 
   Tmp2 = DAG.getCALLSEQ_END(Chain, DAG.getIntPtrConstant(0, dl, true),
                             DAG.getIntPtrConstant(0, dl, true), SDValue(), dl);